In conventional fuel injection systems, the fuel injectors may be mechanically, hydraulically, or electrically actuated. In hydraulically-actuated systems, the pumping assembly which periodically causes fuel to be injected into the engine cylinders is hydraulically driven by pressurized actuating fluid which is selectively communicated to the pumping assembly by an electronically-controlled valve. One example of a hydraulically-actuated, electronically-controlled fuel injection system is disclosed in U.S. Pat. No. 5,121,730 to Ausman, et al.
In mechanically-actuated systems, the pumping assembly is mechanically coupled to a cam driven by the engine so that the pumping assembly is actuated in synchronism with the rotation of the cam. The precise timing and duration of injection is determined by an electronically-controlled valve associated with the pumping assembly. Typically, the electronically-controlled valve is a solenoid valve.
In an engine in which such a fuel injection system is incorporated, the emissions, such as particulate emissions and NOx emissions, generated by the engine may change over a relatively long period of time, such as a year or more, as the engine ages. Since there are relatively strict government standards which limit the amount of emissions the engine may generate, the fact that the emissions may change over long periods of time is a disadvantage.